Biology – Kevin Dees

Chapter 17From Gene to Protein

Biology – Kevin Dees

DNA

• The information molecule• Sequences of bases is a code• DNA organized in to chromosomes• Chromosomes are organized into genes

• What do the genes actually say???

Biology – Kevin Dees

Reflecting back to Mendel

• Mendel understood characters and basic inheritance patterns– Example - pea’s stem length

• Dominant tall; recessive short– Mendel did not understand the physiological

basis for this difference• Short pea plants lack gibberellins, hormones which

stimulate the elongation of stems• Why do short pea plants lack gibberellins? They

are missing a key protein which serves as an enzyme required in gibberellin's synthesis

Biology – Kevin Dees

In other words…

• DNA gene protein

• Proteins are the link between the genotype and the phenotype

• The process by which DNA directs protein synthesis is known as gene expression

Biology – Kevin Dees

Gene expression

• Two steps

– Transcription – making a copy of the gene in the form of messenger RNA (mRNA)

– Translation - converting the information into the correct sequence of amino acids; ribosomes and transfer RNA (tRNA)

Biology – Kevin Dees

• There are subtle differences between the gene expression process as seen in prokaryotes and eukaryotes – No nucleus in

prokaryotes– Pre-MRNA in

eukaryotes

Biology – Kevin Dees

Process of transcription

• Occurs in the nucleus of eukaryotic cells • mRNA will be synthesized as a copy of a

gene from the DNA template• RNA polymerase enzyme separates the

strands of DNA and adds the RNA nucleotides which correspond to the complementary bases on the DNA template – Remember Uracil replaces Thymine in RNA

molecules!!!

Biology – Kevin Dees

• Transcription begins at special sequences of bases called promotersites

• RNA polymerase begins the synthesis of the mRNA transcript here

• The new mRNA transcript grows in a 5’ 3’ direction

• The RNA polymerase complex moves down the DNA template until the terminator sequence is reached, signaling the end of the gene.

Biology – Kevin Dees

• So, if the mRNA strand grows 5’ 3’

• The DNA template is read 3’ 5’

• In eukaryotes transcription rates may be as fast as 60 nucleotides per second

Biology – Kevin Dees

mRNA is organized into codons

• Sequences of three bases

• Using the four bases (A,U,G,C) there are 64 possible combinations – 43=64

• As we will see, each codon will ultimately code for a particular amino acid– ~20 naturally occurring

amino acids

Biology – Kevin Dees

• In Eukaryotes, this occurs in the nucleus

• The initial mRNA transcript (called pre-mRNA) now must be processed to– remove the non-coding regions– Introns – non-coding regions– Add the cap and tail – signals to the start

and end of the mRNA molecule

Biology – Kevin Dees

Translation

Literally a translation from nucleic acid language…In the form of codons on mRNA

To amino acid languageIn the form of correct sequence of amino acids to make

a polypeptide

In eukaryotes this occurs in the cytoplasm in conjunction with ribosomes and tRNA

Biology – Kevin Dees

Ribosomes• Constructed of ribosomal RNA and proteins• Large and small ribosomal subunits• mRNA binding site• A site• P site• E site

Biology – Kevin Dees

Transfer RNA

• Is responsible for bringing the correct amino acid to the ribosome-mRNA complex

• tRNA will bind to specific codons on mRNA using anticodons

Biology – Kevin Dees

Creation of a polypeptide• tRNA binds to A site• GTP GDP

– Like ATP– Loses phosphate;

releases energy• Dehydration synthesis• peptide bond forms

between adjacent amino acids

• Ribosome moves• Growing polypeptide and

tRNA now at P site, A site open and new tRNA with amino acid joins

• GTP GDP• Ribosome moves again• tRNA now on exit site and

is removed

Biology – Kevin Dees

Final step

• A release factor binds to the A site (at a stop codon on mRNA) and hydrolyzes the bond between the tRNA in the P site and the last amino acid in the polypeptide chain

Biology – Kevin Dees

• Overview for review!!!

Biology – Kevin Dees

Mutations and the effect on genes

• Mutations are changes in the genetic material of a cell

• Large scale mutations which compromise large portions of chromosomes and many genes may cause a host of problems

• Point mutations are chemical changes in just one base-pair of a gene

Biology – Kevin Dees

Sickle celled anemia is can be traced back to such a point mutation